Strict Suboptions Ian McIntosh Toronto IBM Lab

1. Strict SuboptionsIan McIntoshToronto IBM Lab

2. Agenda • Why Strict Suboptions Are Needed • Overall Suboptions Design • General Suboptions • Detail Suboptions • Examples • Pragmas and Directives • Interacting and Related Options • Options Listing • Q&A

3. Why Strict Suboptions Are Needed • Using nostrict allows many programs to run faster. • Using strict is sometimes necessary for correctness. • Some programs would run faster with nostrict for some compiler decisions but need strict for others. • nostrict-related problem investigation would be easier with more detailed controls.

4. Overall Suboptions Design • Detail Level: • Some users want general high level controls, • other users need very specific low level controls, • so different views of the same controls are needed. • Hundreds of decisions are controlled by strict/nostrict, • so most suboptions will control groups of decisions. • Future versions may need even more detailed suboptions. • Constraints: • strict suboptions control the same things strict did. • Future versions may control more decisions, • so may need more suboptions. • As simple as will meet users’ needs.

5. Overall Suboptions Design • -qstrict -- same as before • -qnostrict -- same as before • -qstrict=all -- same as -qstrict • -qstrict=none -- same as –qnostrict • -qstrict=X -- be strict about X(X is any strict suboption) • -qstrict=noX -- be relaxed about X • -qstrict=X:Y:Z -- be strict or relaxed about X, Y and Z • (X and/or Y and/or Z may have “no” prefix) • -qstrict=X\ -- be strict or relaxed about X, Y and Z-qstrict=Y:Z (X and/or Y and/or Z may have “no” prefix)

6. General Suboptions • -qstrict=all -- be strict or • -qstrict=none -- relaxed about all changes • -qstrict=precision -- be strict or • -qstrict=noprecision-- relaxed about changing precision • -qstrict=exceptions -- be strict or • -qstrict=noexceptions -- relaxed about changing exceptions (whether more or less or moved) • [no]exceptions does not control everything that could produce different results, so different exceptions are possible even with –qstrict=noexceptions. • Each general suboption controls decisions itself,and also controls nested suboptions.

7. General Suboptions • -qstrict=ieeefp -- be strict or • -qstrict=noieeefp -- relaxed about violating IEEE 754 • [no]ieeefp controls individual operations defined by IEEE 754, not how operations interact or are ordered. • Detail suboptions allow controlling specific aspects of [no]ieeefp.Most operations are affected by multiple detail suboptions, and also by [no]exceptions. • -qstrict=order -- be strict or • -qstrict=noorder -- relaxed about operation order • [no]order controls the order between operations, as defined by language semantics, not how each operation is implemented. • Detail suboptions allow controlling specific aspects of [no]order.

8. Detail Suboptions – ieeefp group • -qstrict=nans -- be strict or • -qstrict=nonans -- relaxed about calculations with NaNs eg, x+0. => x is wrong if x is a NaN • -qstrict=infinities -- be strict or • -qstrict=noinfinities -- relaxed about calculations with infinities eg, x*0. => 0. is wrong if x is an infinity • -qstrict=subnormals -- be strict or • -qstrict=nosubnormals -- relaxed about calculations with subnormals eg, x/10. => x*.1 overflows if x is subnormal • -qstrict=zerosigns -- be strict or • -qstrict=nozerosigns -- relaxed about calculations with sign of zero eg, (-a)-b*c => –(a+b*c) may give the wrong zero • -qstrict=operationprecision -- be strict or • -qstrict=nooperationprecision-- relaxed about operation precision eg,x/5. => x*.2 is not precise Most operations need more than one of these; some also need noexceptions.

9. Detail Suboptions – order group • -qstrict=association -- be strict or • -qstrict=noassociation -- relaxed about changing association order • eg, a+b+c, (a+b)+c, a+(b+c), (a+c)+b may give different results • -qstrict=reductionorder -- be strict or • -qstrict=noreductionorder -- relaxed about reductions like dot product changing the order of evaluation (and hence the order of rounding) • -qstrict=guards -- be strict or • -qstrict=noguards -- relaxed about moving variable use past if / for / while / call eg, if(k>=0) x=y[k] => t=y[k]; if(k>=0) x=t is faster but risky if k is not a valid subscript

10. Detail Suboptions – miscellaneous • -qstrict=library -- be strict or • -qstrict=nolibrary -- relaxed about replacing library functions • eg, libm sin(x) => MASS sin(x) may give different results • -qstrict=constructcopy -- be strict or • -qstrict=noconstructcopy -- relaxed about constructing Fortran REAL arrays via a temporary array to ensure atomic construction or not in case of exceptions

11. Suboption Grouping • all and none include: • everything. • precision includes: • subnormals, operationprecision, association, reductionorder, library, • and controls some things itself. • exceptions includes: • nans, infinities, subnormals, guards, library, constructcopy, • and controls some things itself. • ieeefp includes: • nans, infinities, subnormals, zerosigns, operationprecision. • order includes: • association, reductionorder, guards.

15. Examples • -qstrict=all:nooperationprecision:noreductionorder • Be strict about everything, • except relaxed about operationprecision, part of what’s needed to allowx / loop_constant => x * (1 / loop_constant),which is needed to allow faster MOD(note this allows other changes too), • and relaxed about reductionorder, to allow recognizing dot product and similar reductions and generating faster parallelized code, without allowing other reordering.

16. Examples

17. Examples dp = 0. do i = 1, n dp = dp + a(i) * b(i) end do

18. Examples

19. Pragmas and Directives • XL C/C++ #pragma option_override • #pragma option_override (fname,"opt(strict,all,noreductionorder)") • XLF @PROCESS directive • @PROCESS STRICT(ALL, NOREDUCTIONORDER) These allow: • Putting required suboptions in the source file. • Having suboptions control specific functions or subroutinesinstead of everything in all the files compiled together.

20. Interacting Options • -qstrict sets –qfloat=nofltint • -qnostrict sets –qfloat=fltintso do –qstrict=[no]operationprecision or/and [no]exceptions • -qstrict sets –qfloat=norsqrt • -qnostrict sets –qfloat=rsqrtso do –qstrict=[no]infinities or/and [no]operationprecision or/and [no]exceptions • -qstrict sets –qfloat=rngchkso do –qstrict=[no]nans or [no]infinities or [no]zerosignsor [no]exceptions • -qfloat=relaxaffects some of the same things as –qstrict=noieeefp

21. Related Options • Other options which strict/nostrict users may have some interest in: -qsmp • -qfloat=maf-qessl • -qfloat=strictnmaf-qhot=vector • -qfloat=rndsngl-qhot=simd • -qfloat=hsflt-qenablevmx • -qfloat=hscmplx (xlf) -qalias=… • -qfloat=hssngl-qassert=… (xlf) • -qfloat=single (xlf) -qxlf90=signedzero (xlf) • -qfloat=fltint-qxlf2003=signdzerointr (xlf) • -qfloat=nans-qstrictieeemod (xlf) • -qfloat=spnans-qport=sce (xlf) • -qfloat=rrm-qswapomp (xlf) • -qstrict_induction-qfloat=fenv (xlf)

22. Options Listing • -qstrict => STRICT (as before) • -qnostrict => NOSTRICT (as before) • -qstrict=all => STRICT=ALL • -qstrict=none => STRICT=NONE • -qstrict=X:Y => STRICT=[ALL|NONE]:X:Y:all “no” suboptions • -qlistopt => STRICT=all strict suboptions • Fortran listing syntax is STRICT(X,Y,Z) not STRICT=X:Y:Z. • With –qsaveopt, options are also put in the object file.

23. Questions and Answers